Array of variable length in a structure

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In C99 and later, you can have a (one-dimensional) flexible array member (FAM) at the end of a structure:

§6.7.2.1 Structure and union specifiers

¶18 As a special case, the last element of a structure with more than one named member may
have an incomplete array type; this is called a flexible array member. In most situations,
the flexible array member is ignored. In particular, the size of the structure is as if the
flexible array member were omitted except that it may have more trailing padding than
the omission would imply. However, when a . (or ->) operator has a left operand that is
(a pointer to) a structure with a flexible array member and the right operand names that
member, it behaves as if that member were replaced with the longest array (with the same
element type) that would not make the structure larger than the object being accessed; the
offset of the array shall remain that of the flexible array member, even if this would differ
from that of the replacement array. If this array would have no elements, it behaves as if
it had one element but the behavior is undefined if any attempt is made to access that
element or to generate a pointer one past it.

That means you could write:

typedef struct image
{
    int width;
    int height;
    struct pixel pixels[];
} image;

but you would have to do the 2D-to-1D index mapping yourself. You also have to be careful how you allocate the structure (of necessity, it will be allocated by malloc() as otherwise the size of the array will be zero).

Note the addition of a name for the typedef (I chose image to match the structure tag, but you can choose any other name you like). A typedef with no name is ‘valid’ but not useful — you could omit typedef and get the same result.

You might use:

image *ip = malloc(sizeof(image) + width * height * sizeof(struct pixel));

if (ip != 0)
{
    ip->width = width;
    ip->height = height;
    for (int i = 0; i < height; i++)
    {
        for (int j = 0; j < width; j++)
            ip->pixels[i*width + j] = default_pixel_value;
    }
    …use ip…
    free(ip);
}

I’m not sure that there’s a good way to get a 2D array as the FAM.

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